FIG. 6 shows a known reflow soldering method of joining electronic parts, lead members or the like to a circuit board using a conventional tin-lead solder paste (see, for example, JP-A No. 2002-362003).
First, a metal mask 1 is prepared which has openings 11 at portions thereof corresponding to the positions of electrodes 21 on a circuit board 2 as shown in FIG. 6(a). The metal mask 1 is attached to the circuit board 2 as shown in FIG. 6(b), and tin-lead solder paste 3 is placed on the metal mask 1. Subsequently, a printing squeegee 4 is moved on the metal mask 1 from one end thereof toward the other end, whereby the openings 11 are filled with the tin-lead solder paste 3. The metal mask 1 is then peeled off, whereby a tin-lead solder paste pattern 3a is printed on the circuit board 2 over the electrodes 21 thereon as shown in FIG. 6(c). Electronic parts 5, a lead member 6, etc. are thereafter placed on the solder paste pattern 3a printed on the board 2 as shown in FIG. 6(d), and the board 2 is passed through a reflow furnace for soldering.
Urethane, metal or the like is used as the material for the printing squeegee 4. Because the squeegee of urethane, if used, has the problem that the urethane is scraped off by an edge portion of the metal mask to leave fragments on the board or solder paste pattern, a squeegee of metal is generally used.
It is desired that the circuit boards for use in electronic devices exhibit higher performance, have a multiplicity of functions and be compacted, so that many electronic parts must be mounted on the circuit board using reflow soldering techniques. The above-mentioned tin-lead solder paste has heretofore been used generally for reflow soldering.
However, if electronic devices and circuit boards having parts mounted thereon are discarded in the natural environment after use without being suitably disposed of, there arises the problem that lead, which is toxic heavy metal, will flow out in the form of soluble compounds by being exposed to acid rain, not only adversely affecting the global environment but also exerting influence on animals, plants and human bodies by ingressing into underground water or other water systems. For this reason, it is strongly desireable to use lead-free solders.
Accordingly, tin-silver, tin-silver-copper, tin-bismuth, tin-zinc, tin-antimony and like lead-free solders have been developed, and solder pastes containing such lead-free solders are placed into use. Especially tin-silver and tin-silver-copper solders are usable in place of the tin-lead solder with reliability as high as is conventionally available since silver is stable.
The lead-free solver paste is prepared by mixing a lead-free solder powder consisting mainly of tin with a flux. However, whereas the tin-lead solder has a melting point of about 183° C., the tin-silver or tin-silver-copper lead-free solder has a high melting point of about 220° C., so that if the same flux as the one used in the tin-lead solder is used, the flux vaporizes off before the solder of high melting point melts in the reflow soldering process, and it is impossible to perform satisfactory soldering.
Accordingly, the flux generally in use for the tin-silver or tin-silver-copper lead-free solder paste of high melting point is a composition comprising rosin or modified rosin serving as a base, and a solvent, activator, thixotropic agent and other additives added to the base (see, for example, JP-A No. 2003-10996).
In the case where such a tin-silver, tin-silver-copper or like lead-free solder paste of high melting point is to be printed on circuit boards, the lead-free solder paste is filled into rectangular openings 11 formed in a metal mask 1 as shown in FIG. 3, and a printing squeegee 4 of metal is thereafter moved along the metal mask 1 as shown in FIG. 7(a). If the openings 11 are each so large as to exceed 5 mm.sup2 in area, great frictional heat is liable to occur between the printing metal squeegee 4 and edge portions 15 of the opening 11 in the metal mask 1 shown in FIG. 7(b). Further because the opening 11 is rectangular in shape, the lead-free solder paste filled in is low in fluidity and is liable to be heated to a high temperature, especially at the edge portion 15. However, the flux incorporated in the lead-free solder paste has characteristics to undergo a chemical change and deteriorate (oxidize) when exposed to a high temperature, so that the flux markedly deteriorates (oxidizes) as a result of the frictional heat to result in impaired solder wettability. This seriously lowers the bond strength between a lead member or like large part and the circuit board, entailing the problem that the part becomes removed from the circuit board when subjected, for example, to external pressure or stress.
Furthermore, if the opening 11 of the metal mask 1 has an increased size, it becomes difficult to fill the opening 11 with the solder paste uniformly. Further the reflow soldering process wherein a board 2 having a solder paste pattern 3a printed thereon and carrying parts on the pattern as shown in FIG. 6(d) is passed through a reflow furnace to thereby melt the solder paste and solder the parts onto the board 2 has the problem that the parts are moved and become shifted from the specified positions by virtue of a self-alignment effect produced when the solder paste is melted.